Fractionation trays are widely used in the petrochemical and petroleum refining industries to promote the multistage vapor-liquid contacting performed in fractional distillation. They are mounted in a horizontal position in a vertical pressure vessel referred to as a fractionation column. The normal configuration of a fractionation column includes about 10 to 120 individual trays. Normally each tray is the same. The trays are mounted horizontally at uniform vertical distances referred to as the tray spacing of the column. This distance may vary within different parts of the column but is normally considered constant. The trays are normally supported in part by rings attached to the inner surface of the column, with an overlap providing a seal at the edge of the tray.
Vapor generated at the bottom of the column rises through the tray, which supports a quantity of liquid. The passage of the vapor through the liquid generates a layer of bubbles referred to as froth. The high surface area of the froth helps to quickly establish a compositional equilibrium between the vapor and liquid phases on the tray. The vapor loses less volatile material to the liquid and thus becomes slightly more volatile as it passes upward through each tray. The liquid separates from the froth and travels downward to the next lower tray. This froth formation and separation is performed on each tray. Trays therefore perform the two functions of contacting the rising vapor with liquid and then allowing the two phases to separate and flow in different directions. When the steps are performed a suitable number of times, the process can lead to highly effective separation of chemical compounds based upon their relative volatility.